Germinal centers (GCs) are focal sites of B cell proliferation and differentiation. One major function of GCs during the immune response is to enhance clonal expansion and selection of high affinity B cells (affinity maturation). The elimination of low affinity B cell clones is a protective mechanism to deter the emergence of self-reactive antibodies. T cell assistance is important for certain steps in GC B cell differentiation and selection, and in the absence of T cells, GCs are very rare and short lived. The exact mechanism by which T cells contribute to the GC response is still not clear. The overall goal of this proposal is to determine how helper T cells influence B cells throughout the GC response. This will be achieved by addressing the following specific aims: 1. To determine the role of affinity in regulating founder B cell access to the nascent germinal center. 2. Determine the phenotype, function, and dynamic behavior of GC-resident T cells. 3. To investigate the role of follicular T cells in maintaining the GC reaction and in limiting the emergence of autoreactive B cell clones. The major methods to be employed include using novel mice expressing photoactivatable green fluorescent protein (PA-GFP), two-photon laser scanning microscopy (TPLSM), and microarray. PA-GFP can be used to label cell populations in distinct anatomic regions within the GC in situ by TPLSM. These cells can then be identified and thoroughly phenotyped by flow cytometry and microarray. Upon successful completion of the proposed studies, the expertise, technical and conceptual approach regarding T cell biology will be multi-faceted and applicable towards many avenues of human health. Thereby, helping to ensure a successful transition from a post- doctorate to an independent investigator position. The implications of these studies have major health-related consequences, as self-reactive antibodies mediate many autoimmune diseases. The proposed studies here will determine whether T cells, through their interactions with GC B cells, are essential in regulating the emergence of self-reactive antibodies.